Process to prepare pressure-sensitive hybrid composite latex adhesives

ABSTRACT

The invention is directed to an improvement in a process for preparing an aqueous emulsion of pressure-sensitive adhesive based upon acrylic esters having a good balance of adhesive and cohesive properties and to the resulting emulsion. In the basic process, a pressure-sensitive adhesive formulation comprised of at least one ester of acrylic or methacrylic acid is polymerized in the presence of water and an emulsifier thereby forming an emulsion polymerized pressure-sensitive adhesive. The improvement resides in effecting the polymerization of said pressure-sensitive adhesive formulation comprised of at least one ester of acrylic or methacrylic acid in the presence of from 5 to 30 percent by weight of a styrene containing polymer containing at least 80 percent by weight styrene.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a division of U.S. application Ser. No.10/262,249, filed on Sep. 30, 2002, to be issued as U.S. Pat. No.6,747,084 on Jun. 8, 2004.

BACKGROUND OF THE INVENTION

[0002] Pressure-sensitive adhesives (sometimes referred to as PSA) whichare permanently tacky in dry form at room temperature are widely usedfor making labels and tapes which can be applied to a variety ofsubstrates and adhere on application of slight pressure. They are alsoused for laminating polymeric films such as poly(vinyl chloride) andpolyester Mylar, silicone coated papers, and film release liners forforming decals and other related products.

[0003] Water based pressure-sensitive adhesives are of interest becauseof their low VOC emissions. Although the performance is not equivalentto solvent based pressure-sensitive adhesives, they satisfy emissionstandards and are easy to process. Common types of pressure-sensitiveadhesives, both water based and solvent based, are derived from acrylicester based copolymers, such as alkyl acrylate and alkyl methacrylatecopolymers.

[0004] The following patents and articles are representative of acrylicbased pressure-sensitive adhesives:

[0005] Hidalgo, et al. “Polystyrene(1)/poly(butyl acrylate-methacrylicacid)(2) core-shell emulsion polymers. Part Il: Thermomechanicalproperties of latex films,” Colloid and Polymer Science, 1992, Vol. 270,pages 1208-1221, disclose the formation of polystyrene/poly(butylacrylate-methacrylic acid) latexes by a two stage process. Initially, apolystyrene seed is prepared and then the butyl acrylate and methacrylicacid polymerized in the presence of the seed forming a core/shellpolymer in a ratio of ⅔.

[0006] EP 0 593231 A1 discloses the formation of pressure-sensitiveacrylic adhesives by the addition of low molecular weight (<7,000)ethylene oxide-block-propylene oxide copolymer surfactants to acrylicpressure-sensitive adhesives for the purpose of improving lowtemperature adhesion. These pressure-sensitive adhesives are based upon2-ethylhexyl acrylate and acrylic and methacrylic esters of C₄₋₁₂alkanols, such as butyl acrylate.

[0007] U.S. Pat. No. 6,225,401 discloses filterable aqueous dispersionsof pressure-sensitive adhesive suited for labels formed bycopolymerizing acrylic or methacrylic esters in the presence of aninhibitor. A wide variety of hardening comonomers can be included in theemulsion polymerization process and these include the styrenes,acrylonitrile, vinyl esters, and so forth.

[0008] U.S. Pat. No. 6,254,985 discloses aqueous emulsions ofpressure-sensitive adhesives base upon esters of acrylic and methacrylicacid. The patentees disclose the use of an emulsifier consisting of atleast 5% by weight of aromatic carbon atoms, typically including atleast two sulfonate groups to improve adhesion and cohesion.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention is directed to an improvement in a processfor preparing an aqueous emulsion of a pressure-sensitive adhesive basedupon acrylic esters which have a good balance of adhesive and cohesiveproperties and to the resulting emulsion. In the basic process, apressure-sensitive adhesive formulation comprised of at least one esterof acrylic or methacrylic acid is polymerized in the presence of waterand an emulsifier thereby forming an emulsion polymerizedpressure-sensitive adhesive polymer. The improvement resides ineffecting the polymerization of said pressure-sensitive adhesiveformulation comprised of an ester of acrylic or methacrylic acid and astyrene containing polymer containing at least 80 percent by weightstyrene, said styrene containing polymer present in an amount of from 5to 30 percent by weight of the pressure-sensitive adhesive formulation.

[0010] Significant advantages of the process and product can be realizedand they include:

[0011] an ability to include a small proportion of a low cost fillerpolymer into an aqueous pressure-sensitive adhesive based upon acrylicand methacrylic esters without adversely affecting the adhesiveproperties;

[0012] an ability to overcome deficiencies in film formation from blendsof polymers, e.g., blends of polystyrene and acrylic and methacryliccopolymers;

[0013] an ability to include a low cost “filler” into apressure-sensitive adhesive by a simple method without the need forspecial equipment;

[0014] an ability to prepare a pressure-sensitive adhesive with high Tgpolystyrene filler in one reaction, and in a single reactor, whilemaintaining the performance advantages of the pressure-sensitiveadhesive including those formed by the blend method; and,

[0015] an ability to eliminate the need for the high Tg polystyrenelatex to be of a specific particle size, expensive macromers, andminimum amounts of surfactants.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Emulsion polymerization of a pressure-sensitive adhesiveformulation comprised of esters of acrylic and methacrylic acidincluding ethylenically unsaturated monomers to produce aqueous basedpressure-sensitive adhesive polymer emulsions is well known. Arepresentative pressure-sensitive adhesive formulation is comprised ofan aqueous polymer dispersion wherein the polymer is comprised ofpolymerized units, based on the total weight of units, of (a) from 60 to95% by weight of at least one C₆₋₁₂ alkyl acrylate; (b) from 0 to 10% byweight of an ethylenically unsaturated compound having a glasstransition temperature of above 0° C. and contain no functional groupsother than ethylenically unsaturated group; (c) from 0 to 10% by weightof an ethylenically unsaturated compound having at least one acid oracid anhydride group; and (d) from 0 to 20% by weight of a furtherethylenically unsaturated compound; the weight percentages based on thetotal weight of polymer. Typically, unsaturated compounds in group (c)and (d) are included at less than 5% each, when used. Thus, compounds ingroup (c) and (d) comprise a small proportion of the pressure-sensitiveadhesive.

[0017] Particularly suitable alkyl acrylates in group (a) are2-ethylhexyl acrylate, octyl acrylate, decyl acrylate or dodecylacrylate.

[0018] Monomers in group (b) can include methyl methacrylate, methylacrylate, n-butyl acrylate and tert-butyl acrylate; vinyl esters ofC₁₋₂₀ carboxylic acids such as vinyl laurate, stearate, propionate, thevinyl ester of Versatic acid, and vinyl acetate; vinyl aromatics such asstyrene, and so forth. Methyl methacrylate is preferred.

[0019] Examples of group (c) monomers can include acrylic andmethacrylic acid, maleic acid, or maleic anhydride. Group (d) monomerscan include C, to C₁₀ hydroxyalkyl (meth)acrylates.

[0020] There are two mechanisms in the process for forming thepressure-sensitive adhesive including the styrene containing polymer,e.g., polystyrene filler. In one mechanism, a styrene containing polymeris dissolved in a pressure-sensitive adhesive formulation comprised of amixture of monomers. The resulting solution, then, is emulsified withsurfactants and water and, with the aid of energy supplied by high shearmixing, converted to a stable emulsion of relatively small particle sizeparticles. The resulting emulsion is polymerized by emulsionpolymerization. In a second method, a seed latex of styrene polymer isprepared by emulsion polymerization and the pressure-sensitive adhesiveformulation emulsion polymerized in the presence of the seed latex. Thesecond method has the advantage of allowing for reduced emulsifier inthe final product, elimination of high shear mixing and allowing thereaction to be carried out in a single reactor.

[0021] The styrene containing polymer is one containing at least 80% byweight styrene and typically one containing 100% styrene by weight.Optional monomers that may be included in producing styrene containingpolymers are α-methyl styrene, ρ-methylstyrene, acrylonitrile,methacrylonitrile, methacrylonitrile, methyl methacrylate and tracelevels of other monomers leading to a high Tg polymer, at least 80° C.

[0022] The styrene copolymer is incorporated into the pressure-sensitiveadhesive in an amount from 5 to 30% by weight, which includes the basepressure-sensitive adhesive polymer and the styrene containing polymer.Levels above about 30% by weight detract from the performance of thepressure-sensitive adhesive. Levels below about 5% by weight, althoughnot adversely affecting the properties of the pressure-sensitiveadhesive, do not afford the low cost advantages. Preferably, the levelof styrene copolymer is from 10 to 20% by weight.

[0023] Many of the pressure-sensitive adhesive formulations includestyrene as a comonomer. Usually, styrene is included at low levels,e.g., below 5% by weight. High levels of styrene in thepressure-sensitive adhesive formulation lead to unacceptably high Tg ofthe pressure-sensitive adhesive polymer. Such high levels of styreneadversely affect loop tack in the PSA. Accordingly, the ability toincorporate an amount of styrene via copolymerization into thepressure-sensitive adhesive polymer equal to that where thepressure-sensitive adhesive formulation is polymerized in the presenceof styrene containing polymer as in the hybrid composite is not anacceptable option.

[0024] The pressure-sensitive adhesive formulation is designed to leadto a hybrid composite having a Tg of −25° C. to −90° C., preferably 40°C. to −75° C. and a loop tack adhesion value greater than 1 pound perlinear inch (pli); preferably greater than 1.5 pli, according toPressure-sensitive Test Council (PSTC) test method PSTC-5, tested onstainless steel panel. Even though the Tg is not as well defined as in asingle polymer, the Tg of the composite affords an approximation.

[0025] Polymerization of the seed latex, as well as thepressure-sensitive adhesive formulation can be initiated by thermalinitiators or by a redox system. A thermal initiator is typically usedat temperatures at or above about 70° C. and redox systems are preferredat temperatures below about 70° C. The amount of thermal initiator usedin the process is 0.1 to 3 wt %, preferably not more than about 0.5 wt%, based on total monomers. Thermal initiators are well known in theemulsion polymer art and include, for example, ammonium persulfate,sodium persulfate, and the like. The amount of oxidizing and reducingagent in the redox system is about 0.1 to 3 wt %. Any suitable redoxsystem known in the art can be used; for example, the reducing agent canbe a bisulfite, a sulfoxylate, ascorbic acid, erythorbic acid, and thelike. The oxidizing agent can include hydrogen peroxide, organicperoxide such as t-butyl peroxide, persulfates, and the like.

[0026] Chain transfer agents, well known in the aqueous emulsionpolymerization art; are typically used but are not required. Examplesinclude dodecyl mercaptan, mercaptocarboxylic acids, and esters ofmercaptocarboxylic acid. The chain transfer agent is added at levels ofabout 0.01 to 0.5 wt %, preferably 0.02 to 0.15 wt %, based on theweight of monomers.

[0027] Effective emulsion polymerization reaction temperatures rangefrom about 50 to about 100° C. depending on whether the initiator is athermal or redox system.

[0028] The emulsifying agents which are suitably used are typicallyanionic, nonionic or blends thereof. Suitable nonionic emulsifyingagents include polyoxyethylene condensates. Polyoxyethylene condensatesmay be represented by the general formula: R—(CH₂CH₂O—)_(n)H where R isthe residue of a fatty alcohol containing 10 to 18 carbon atoms, analkylphenol, a fatty acid containing 10 to 18 carbon atoms, an amide, anamine, or a mercaptan, and where n is an integer of 1 or above. TheIgepal surfactants are members of a series ofalkylphenoxy-poly(ethyleneoxy)ethanols having alkyl groups containingfrom about 7-18 carbon atoms, and having from about 4 to 100 ethyleneoxyunits, such as the octylphenoxy poly(ethyleneoxy)ethanols, nonylphenoxypoly(ethyleneoxy)ethanols, and dodecylphenoxy poly(ethyleneoxy)ethanols.Examples of nonionic surfactants include polyoxyalkylene derivatives ofhexitol (including sorbitans, sorbides, mannitans, and mannides)anhydride, partial long-chain fatty acid esters, such as polyoxyalkylenederivatives of sorbitan monolaurate, sorbitan monopalmitate, sorbitanmonostearate, sorbitan tristearate, sorbitan monooleate and sorbitantrioleate.

[0029] Suitable anionic emulsifying agents include the monovalent saltsof the sulfates of the above mentioned nonionics, mono or disodium saltsof sulfosuccinates half esters or diesters, sodium salts of alkylbenzenesulfonates. A single emulsifying agent can be used, or the emulsifyingagents can be used in combination. When combinations of emulsifyingagents are used, it is advantageous to use a relatively hydrophobicemulsifying agent in combination with a relatively hydrophilic agent. Arelatively hydrophobic agent is one having a cloud point in 1% aqueoussolution below 190° F. (88° C.) and a relatively hydrophilic agent isone having a cloud point in 1% aqueous solution of 190° F. (88° C.) orabove. The concentration range of the total amount of emulsifying agentsuseful is from 0.5 to 5% based on the aqueous phase of the latexregardless of the solids content. The surfactant package is typicallyused in an amount of from 2 to 7 wt % of the emulsions.

[0030] An alkaline buffering agent of any convenient type that iscompatible with the stabilizing agent may be used if it is desired tomaintain the pH of the system at a desired value. The amount of bufferis generally about 0.1 to 0.5 wt % based on the monomers.

[0031] The following examples are intended to illustrate embodiments orthe invention and are not intended to restrict the scope thereof.

EXAMPLE 1 PRESSURE-SENSITIVE ACRYLIC WITH 20% POLYSTYRENE DISSOLVED ANDTHEN EMULSIFIED Polymerization Procedure

[0032] Emulsion polymerization of a pressure-sensitive adhesive wascarried out in conventional manner. More specifically, a 1 gallonreactor was purged with nitrogen and then an “Initial Charge” includinga fraction of the monomers employed in the pressure-sensitive adhesivewas added. The contents were heated to 80° C. Polymerization of themonomers in the initial charge were effected by the addition of a 2.73%solution of sodium persulfate (1.75 ml) over 10 minutes at a rate of0.175 g/min.

[0033] A polymerizable emulsion mix was formed by mixing the“Pressure-Sensitive Adhesive Formulation” with the “Emulsifier” in ahigh shear mixer. After initiation, the initiator, polymerizableemulsion mix, and buffer were added over a period of about 4 hours.After addition of the initiator and polymerizable emulsion mix, thereaction was maintained for an additional 15 min at 80° C. The reactioncontents were cooled to 75° C. and 2 ml of 1% iron was added. To finishthe reaction, redox delays were added, initially 4 ml of each was added,and the reaction held for 30 min. This procedure was repeated asnecessary until the free monomer was reduced to below 0.1%. InitialCharge Component Amount (g) 1. DI water 44.3 2. Sodium pyrophosphate 3%42.3 3. Emulsifier K30 (29.9%) 0.317 4. Aerosol A102 (32%) 2.45 5.2-Ethylhexyl acrylate 28.05 6. Acrylic acid 0.15 7. Styrene 0.68 8.Methyl methacrylate 2.8

[0034] Pressure-Sensitive Adhesive Formulation Component % ComponentAmount (g) Methyl methacrylate 1.74 10.45 2-Ethylhexyl acrylate 72.7436.25 Acrylic acid 97% 0.39 2.34 Styrene 1.48 8.89 Vinyl acetate 2.0812.48 Polystyrene 20 120 2-Hydroxypropyl acrylate 1.6 9.59 Total 100 600

[0035] Emulsifier Component g 1. D.I. water 282 2. Sodium VinylSulfonate. (25% nv) 10.32 3. Aerosol A102 (32% nv) 19.00 Total 311.32

[0036] Initiator 1. DI water 91 g 2. sodium persulfate  9 g Total 100 g 

[0037] Buffer 1. di water 38.36 g 2. sodium citrate 11.64 g Total   50g.

[0038] Redox Delays 1. DI. Water 62.61 g 2. t-BHP (70%)  1.08 g 1. DIwater 62.42 g 2. SFS  0.61 g

[0039] Final Properties of Pressure-Sensitive Adhesive pH  5.0 % solids55.2 Viscosity at 20 rpm, #2 spindle 170 cps

EXAMPLE 2 PRESSURE-SENSITIVE ADHESIVE FORMULATION POLYMERIZED IN THEPRESENCE OF POLYSTYRENE SEED LATEX Polymerization Procedure

[0040] The polymerization procedure of Example 1 was followed. Ingeneral terms the following steps were implemented:

[0041] 1. Reactor was purged with nitrogen.

[0042] 2. First stage: added initial charge consisting essentially ofstyrene as the monomer and heated to 78° C. Added initiator (9% solutionof sodium persulfate) 4.4 grams. Waited for exotherm. In this firststage, a polystyrene seed latex was formed.

[0043] 3. After the first stage and the formation of polystyrene seedlatex, the initiator (3.6%), buffer, and polymerizable emulsion mix wereadded over about 4 hours.

[0044] 4. After the addition of the polymerizable emulsion mix, thereaction product was heated for an additional hour at 90° C.

[0045] 5. When the reaction was complete, the free monomer was checked.The reaction product was cooled to 75° C. and post-treated with theredox delays to reduce the free monomer to below 0.1%.

Initial Charge: Polystyrene Seed Latex Formation

[0046] Material g DI water 290 Versene 220 0.768 Emulsifier K30 (29.9%)1.4 Genopol 1879 (40%) 9.3 Sodium bicarbonate 0.06 Styrene 123.9 Total425.428

[0047] Pressure-Sensitive Adhesive Formulation Monomer % Monomer Amount,g Methyl methacrylate 1.78 12.64 2-Ethylhexyl acrylate 81.43 571.72Acrylic acid 97% 0.51 3.58 Styrene 1.78 12.50 Vinyl acetate 5.29 37.142-Hydroxypropyl acrylate 2.03 14.25 Butyl acrylate 7.18 50.41 Total 100702.24

[0048] Emulsifier Component Amount, g D.I. water 114.5 Sodium vinylsulfonate (25% v) 17.53 Disponil FES 32 (30% nv) (30% nv) 32.48 AerosolA102 (32% nv) 3.03 Total 167.54

[0049] Initiator 1. DI Water 96.4 g 2. Sodium persulfate  3.6 g Total 100 g

[0050] Buffer 1. DI water 46.12 g 2. Sodium citrate  14.0 g Total 60.12g

[0051] Redox Delays 1. DI. Water 33.9 g 2. t-BHP (70%) 1.08 g 1. DIwater 34.39 g  2. SFS 0.61 g 3. Iron (ferrous sulfate) solution;   2 mlof 1% solution; mixed in first

[0052] Final Properties of Pressure-Sensitive Adhesive pH  4.5 % solids57.3 Viscosity 20 rpm, #3 spindle 950 cps

EXAMPLE 3 EVALUATION OF PRESSURE-SENSITIVE ADHESIVES

[0053] The emulsions of Examples 1 and 2 were used to form the varioustest samples and were compared to emulsion blends which consisted of apressure-sensitive adhesive latex having the formulation of Example 1 orExample 2 (pressure sensitive adhesive formulation only) and apolystyrene latex. The latex samples were adjusted to a pH of 7. Theemulsions were directly coated onto 2 ml Mylar at a coat weight of 25g/m². The resulting tapes were applied to various substrates, e.g.,stainless steel (SS) and low density polyethylene (LDPE), and evaluatedfor peel strength. Some of the samples were evaluated for shear basedupon a test where a ½ inch film square was exposed to a 500 gram weight(referred to as ½×½×500) and another where a 1 inch square was exposedto a 1000 gram weight and evaluated (referred to as 1×1×1000). Anothertest was the measured peel to corrugated cardboard test conducted at 35°F. (2° C.) or 20° F. (−7° C.); referred to as 35 CC and 20 CC,respectively. (The sample preparation and test methods used to evaluatethe adhesives or coatings in the examples were based on industrystandard tests. They are described in publications of the PressureSensitive Tape Council (PSTC), Glenview, Ill.) The results are presentedin Tables 1-5. TABLE 1 Peel Peel Peel Shear SS (pli) LDPE (pli) 35 CC(pli) ½ × ½ × 500 Example 1 2.5 1.25 0.83 2 15% Blend 1.45 0.92 0.92 130% Blend 1.36 0.98 0 1.9 Control 2.4 1.15 2.2 0.5

[0054] Table 1 compares the hybrid composite latex of Example 1 with themethod of latex blending, i.e., blending of a polystyrene latex with acontrol pressure-sensitive adhesive (same monomer composition as setforth in the pressure-sensitive adhesive formulation as in Example 1).The pressure-sensitive adhesive formulation alone was referred to as the“control” and blends of 15% and 30% polystyrene by weight were compared.As can be seen from Table 1, the polystyrene, whether as a composite orblend, and even though deemed a filler, maintains pressure-sensitiveadhesive performance properties of the control pressure-sensitiveadhesive. The Example 1 PSA compares favorably with the control in everyarea except low temperature peel, i.e., 35 CC. It is substantial equalto 35 CC peel for the 15% blend and significantly better than the 30%blended sample.

[0055] If styrene were copolymerized into the pressure-sensitiveadhesive at a 20% level, that would increase the Tg by about 20° C. Itwould be like raising the Tg of the PSA from −60° C. to −40° C. Thatresult would adversely affect the loop tack and possibly otherproperties of the pressure-sensitive adhesive.

[0056] Table 2 shows results using three sources of polystyrene. SamplesA, B and C were prepared in accordance with Example 1. Sample A used ahigh molecular weight commercial polystyrene, sample B employed a lowmolecular weight commercial polystyrene, and Sample C used polystyrenebeverage cups dissolved in monomer. TABLE 2 Shear Peel SS (pli) PeelLDPE (pli) Peel 35 CC (pli) 1 × 1 × 1000 A 2.5 1.25 0.83 25 B 2.4 1.50.85 36 C 2.2 1.4 0.85 74

[0057] All samples gave performance properties that were very similarthus illustrating that the performance properties were not greatlyinfluenced by the differing polystyrene polymers employed.

[0058] Table 3 compares results of the hybrid composite of Example 1including polystyrene, a control of PSA alone, and Example 1 compositewith tackifier. TABLE 3 Peel Peel Peel Shear SS (pli) LDPE (pli) 35 CC(pli) 1 × 1 × 1000 Example 1 2.5 1.25 0.83 25 Example 1 + 4 3 1.1 27Tackifier A Control 2.4 1.15 2.2  7

[0059] As expected, addition of tackifier resin improved the peel valuesof the hybrid composite of Example 1 with little effect on shear.

[0060] In summary, the results show that the method of incorporating astyrene containing polymer, e.g., a polystyrene filler into apressure-sensitive adhesive formulation thereby forming a hybridcomposite latex is suitable for PSA applications. Performance is notadversely affected, and cost is reduced.

[0061] In the preferred method of preparation, i.e., polymerizing thepressure-sensitive adhesive formulation in the presence of a polystyreneseed latex, the polymer filler has an overall positive impact onperformance properties and cost. Performance results for the latex madeby example 2, compared with a PSA control and a blend of PSA withpolystyrene latex, are shown in Table 4. TABLE 4 Peel Peel Peel Shear CC(pli) LDPE (pli) 20 CC (pli) 1 × 1 × 1000 Example 2 1.1 2 1.1 7.9 20%Blend 0.58 1.2 0.95 15.2 Control 1.4 0.9 0.9 6.5

[0062] The preferred example shows some differentiation from the latexblend and is more like the PSA control. As in Table 3, addition of atackifying resin to the 20% blend will improve peel on LDPE andcorrugated, Table 5. In fact, the performance of the example 2 latexcompares favorably with both the tackified blend and a tackified generalpurpose commercial label adhesive, Table 5. TABLE 5 Peel Peel Peel ShearCC (pli) LDPE (pli) 20 CC (pli) 1 × 1 × 1000 Example 2 1.1 2 1.1 7.9 20%Blend + 1.2 2 1.1 4.9 Tackifier A Tackified 1.0 2.6 0.85 26 CommercialGP

What is claimed is:
 1. In a pressure-sensitive adhesive comprised ofemulsion polymerized units of (a) from 60 to 95 percent by weight of atleast one at least one C₆₋₁₂ alkyl acrylate; (b) from 0 to 10 percent byweight of ethylenically unsaturated compounds having a glass transitiontemperature of above 0° C. and contain no functional groups other thanan ethylenically unsaturated group; (c) from 0 to 10 percent by weightof ethylenically unsaturated compound having at least one acid or acidanhydride group; and (d) from 0 to 20 percent by weight of furtherethylenically unsaturated compounds, weight percentages based on thetotal weight of polymer, the improvement which comprises forming saidpressure-sensitive adhesive by emulsion polymerization in the presenceof a styrene containing polymer.
 2. The pressure-sensitive adhesive ofclaim 1 wherein the group (a) monomer includes 2-ethylhexyl acrylate. 3.The pressure-sensitive adhesive of claim 2 wherein the group (b) monomerincludes butyl acrylate or methyl methacrylate.
 4. Thepressure-sensitive adhesive of claim 1 wherein the pressure-sensitiveadhesive is comprised of from 5 to 30 percent by weight styrenecontaining polymer based upon the combined weight of saidpressure-sensitive adhesive and said styrene containing polymer andpolystyrene is the styrene containing polymer.
 5. The pressure-sensitiveadhesive of claim 4 wherein the polystyrene is present in an amount from10 to 20 percent by weight.
 6. The pressure-sensitive adhesive of claim5 wherein said aqueous pressure-sensitive adhesive has a Tg of from −45°C. to −75° C.
 7. In a process for producing a pressure-sensitiveadhesive comprised of emulsion polymerized units of (a) from 60 to 95percent by weight of at least one at least one C₆₋₁₂ alkyl acrylate; (b)from 0 to 10 percent by weight of ethylenically unsaturated compoundshaving a glass transition temperature of above 0° C. and contain nofunctional groups other than an ethylenically unsaturated group; (c)from 0 to 10 percent by weight of ethylenically unsaturated compoundhaving at least one acid or acid anhydride group; and (d) from 0 to 20percent by weight of further ethylenically unsaturated compounds, theweight percentages based on the total weight of polymer, the improvementwhich comprises forming said pressure-sensitive adhesive by emulsionpolymerization in the presence of a styrene containing polymer.
 8. Theprocess of claim 7 wherein the styrene containing polymer is present inan amount from 5 to 30 percent by weight of the pressure-sensitiveadhesive.
 9. The process of claim 8 wherein said pressure-sensitiveadhesive has a Tg of from −45° C. to −75° C.
 10. The process of claim 9wherein the pressure-sensitive adhesive is polymerized in the presenceof a polystyrene latex as the styrene containing polymer.
 11. Theprocess of claim 10 wherein the pressure-sensitive adhesive includespolymerized units of least one ester of acrylic or methacrylic acidselected from the group consisting of a C₁ to C₈ alkyl acrylate or a C₁to C₈ alkyl methacrylate.
 12. The process of claim 11 wherein said C₁ toC₈ alkyl acrylate and C₁ to C₈ alkyl methacrylate is selected from thegroup consisting of methyl methacrylate and 2-ethylhexyl acrylate.